You can order a complete interface, or a professionally manufactured circuit board with programmed and tested microprocessor to build your own RS232 interface

The components for the RS232 interface are quite common. You should have no problem to get them from your component shop or order them online. One SMD component is used. This capacitor should be mounted on the solder side, underneath the microprocessor.

All resistors are 0.25 Watt. All capacitors are 50 Volt minimum, unless specified otherwise